1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine which uses an electrophotographic process.
2. Description of Prior Art
In an image forming apparatus which uses an electrophotographic process, it is desirable that an image is formed stably or an image of a particular density is reproduced on a paper for the same image data. In order to reproduce an image stably, it is known to control development conditions based on toner density before a copy operation is started or when a copy mode is set. The toner density is measured on a toner pattern formed on a photoconductor. It is also known to control a development condition based on the toner density of a toner pattern formed on a photoconductor in predetermined conditions and transferred onto a paper at a predetermined transfer current (for example, Japanese Patent laid open Publication 5-134561/1993). The development condition may be, for example, toner density in a development unit, development bias voltage applied to a surface of development sleeve, development interval or the like.
When the amount of charges on toners decreases due to deterioration or the like, toners on the photoconductor are reversely charged in a transfer part of the copying machine, and toners reversely charged are remained on the photoconductor after the toner image is transferred on a paper. This decreases transfer efficiency. In this case, the transfer efficiency can be optimized by controlling the development condition to increase the amount of toner charges. The reverse charging of toners also occurs when the resistance of the paper onto which a toner image is transferred decreases in the environment of high humidity. In this case, the best transfer efficiency or the transfer current for the maximum density is affected. Therefore, besides the control of the development condition to increase the amount of the toner charges, it may be necessary further to control the transfer current. However, prior art image forming apparatuses do not take into account reverse charging due to a change in the resistance of a paper under high humidity environment. Therefore, they cannot decide whether the reverse charging of toners is caused by decrease in toner charges due to deterioration or by decrease in the resistance of the paper under the high humidity environment. Then, in the high humidity environment, the development condition and image forming conditions including the transfer current cannot be controlled appropriately, and an image cannot be reproduced stably on a paper.